Add hotfix for btrfs extent buffer hangs

2 files changed, 148 insertions, 0 deletions

diff --git a/0003-Btrfs-fix-unwritten-extent-buffers-and-hangs-on-future-writeback-attempts.patch b/0003-Btrfs-fix-unwritten-extent-buffers-and-hangs-on-future-writeback-attempts.patch
new file mode 100644
index 0000000..b6d19c3
--- /dev/null
+++ b/0003-Btrfs-fix-unwritten-extent-buffers-and-hangs-on-future-writeback-attempts.patch
@@ -0,0 +1,145 @@
+From 45fc8773f47b7cbe56caab0e14abf26d1e044e63 Mon Sep 17 00:00:00 2001
+From: Filipe Manana <fdmanana@suse.com>
+Date: Wed, 11 Sep 2019 17:42:00 +0100
+Subject: Btrfs: fix unwritten extent buffers and hangs on future writeback
+ attempts
+
+The lock_extent_buffer_io() returns 1 to the caller to tell it everything
+went fine and the callers needs to start writeback for the extent buffer
+(submit a bio, etc), 0 to tell the caller everything went fine but it does
+not need to start writeback for the extent buffer, and a negative value if
+some error happened.
+
+When it's about to return 1 it tries to lock all pages, and if a try lock
+on a page fails, and we didn't flush any existing bio in our "epd", it
+calls flush_write_bio(epd) and overwrites the return value of 1 to 0 or
+an error. The page might have been locked elsewhere, not with the goal
+of starting writeback of the extent buffer, and even by some code other
+than btrfs, like page migration for example, so it does not mean the
+writeback of the extent buffer was already started by some other task,
+so returning a 0 tells the caller (btree_write_cache_pages()) to not
+start writeback for the extent buffer. Note that epd might currently have
+either no bio, so flush_write_bio() returns 0 (success) or it might have
+a bio for another extent buffer with a lower index (logical address).
+
+Since we return 0 with the EXTENT_BUFFER_WRITEBACK bit set on the
+extent buffer and writeback is never started for the extent buffer,
+future attempts to writeback the extent buffer will hang forever waiting
+on that bit to be cleared, since it can only be cleared after writeback
+completes. Such hang is reported with a trace like the following:
+
+  [49887.347053] INFO: task btrfs-transacti:1752 blocked for more than 122 seconds.
+  [49887.347059]       Not tainted 5.2.13-gentoo #2
+  [49887.347060] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
+  [49887.347062] btrfs-transacti D    0  1752      2 0x80004000
+  [49887.347064] Call Trace:
+  [49887.347069]  ? __schedule+0x265/0x830
+  [49887.347071]  ? bit_wait+0x50/0x50
+  [49887.347072]  ? bit_wait+0x50/0x50
+  [49887.347074]  schedule+0x24/0x90
+  [49887.347075]  io_schedule+0x3c/0x60
+  [49887.347077]  bit_wait_io+0x8/0x50
+  [49887.347079]  __wait_on_bit+0x6c/0x80
+  [49887.347081]  ? __lock_release.isra.29+0x155/0x2d0
+  [49887.347083]  out_of_line_wait_on_bit+0x7b/0x80
+  [49887.347084]  ? var_wake_function+0x20/0x20
+  [49887.347087]  lock_extent_buffer_for_io+0x28c/0x390
+  [49887.347089]  btree_write_cache_pages+0x18e/0x340
+  [49887.347091]  do_writepages+0x29/0xb0
+  [49887.347093]  ? kmem_cache_free+0x132/0x160
+  [49887.347095]  ? convert_extent_bit+0x544/0x680
+  [49887.347097]  filemap_fdatawrite_range+0x70/0x90
+  [49887.347099]  btrfs_write_marked_extents+0x53/0x120
+  [49887.347100]  btrfs_write_and_wait_transaction.isra.4+0x38/0xa0
+  [49887.347102]  btrfs_commit_transaction+0x6bb/0x990
+  [49887.347103]  ? start_transaction+0x33e/0x500
+  [49887.347105]  transaction_kthread+0x139/0x15c
+
+So fix this by not overwriting the return value (ret) with the result
+from flush_write_bio(). We also need to clear the EXTENT_BUFFER_WRITEBACK
+bit in case flush_write_bio() returns an error, otherwise it will hang
+any future attempts to writeback the extent buffer, and undo all work
+done before (set back EXTENT_BUFFER_DIRTY, etc).
+
+This is a regression introduced in the 5.2 kernel.
+
+Fixes: 2e3c25136adfb ("btrfs: extent_io: add proper error handling to lock_extent_buffer_for_io()")
+Fixes: f4340622e0226 ("btrfs: extent_io: Move the BUG_ON() in flush_write_bio() one level up")
+Reported-by: Zdenek Sojka <zsojka@seznam.cz>
+Link: https://lore.kernel.org/linux-btrfs/GpO.2yos.3WGDOLpx6t%7D.1TUDYM@seznam.cz/T/#u
+Reported-by: Stefan Priebe - Profihost AG <s.priebe@profihost.ag>
+Link: https://lore.kernel.org/linux-btrfs/5c4688ac-10a7-fb07-70e8-c5d31a3fbb38@profihost.ag/T/#t
+Reported-by: Drazen Kacar <drazen.kacar@oradian.com>
+Link: https://lore.kernel.org/linux-btrfs/DB8PR03MB562876ECE2319B3E579590F799C80@DB8PR03MB5628.eurprd03.prod.outlook.com/
+Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=204377
+Signed-off-by: Filipe Manana <fdmanana@suse.com>
+---
+ fs/btrfs/extent_io.c | 35 ++++++++++++++++++++++++++---------
+ 1 file changed, 26 insertions(+), 9 deletions(-)
+
+diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
+index db337e53aab3..93900ff87df7 100644
+--- a/fs/btrfs/extent_io.c
++++ b/fs/btrfs/extent_io.c
+@@ -3591,6 +3591,13 @@ void wait_on_extent_buffer_writeback(struct extent_buffer *eb)
+ 		       TASK_UNINTERRUPTIBLE);
+ }
+ 
++static void end_extent_buffer_writeback(struct extent_buffer *eb)
++{
++	clear_bit(EXTENT_BUFFER_WRITEBACK, &eb->bflags);
++	smp_mb__after_atomic();
++	wake_up_bit(&eb->bflags, EXTENT_BUFFER_WRITEBACK);
++}
++
+ /*
+  * Lock eb pages and flush the bio if we can't the locks
+  *
+@@ -3662,8 +3669,11 @@ static noinline_for_stack int lock_extent_buffer_for_io(struct extent_buffer *eb
+ 
+ 		if (!trylock_page(p)) {
+ 			if (!flush) {
+-				ret = flush_write_bio(epd);
+-				if (ret < 0) {
++				int err;
++
++				err = flush_write_bio(epd);
++				if (err < 0) {
++					ret = err;
+ 					failed_page_nr = i;
+ 					goto err_unlock;
+ 				}
+@@ -3678,16 +3688,23 @@ err_unlock:
+ 	/* Unlock already locked pages */
+ 	for (i = 0; i < failed_page_nr; i++)
+ 		unlock_page(eb->pages[i]);
++	/*
++	 * Clear EXTENT_BUFFER_WRITEBACK and wake up anyone waiting on it.
++	 * Also set back EXTENT_BUFFER_DIRTY so future attempts to this eb can
++	 * be made and undo everything done before.
++	 */
++	btrfs_tree_lock(eb);
++	spin_lock(&eb->refs_lock);
++	set_bit(EXTENT_BUFFER_DIRTY, &eb->bflags);
++	end_extent_buffer_writeback(eb);
++	spin_unlock(&eb->refs_lock);
++	percpu_counter_add_batch(&fs_info->dirty_metadata_bytes, eb->len,
++				 fs_info->dirty_metadata_batch);
++	btrfs_clear_header_flag(eb, BTRFS_HEADER_FLAG_WRITTEN);
++	btrfs_tree_unlock(eb);
+ 	return ret;
+ }
+ 
+-static void end_extent_buffer_writeback(struct extent_buffer *eb)
+-{
+-	clear_bit(EXTENT_BUFFER_WRITEBACK, &eb->bflags);
+-	smp_mb__after_atomic();
+-	wake_up_bit(&eb->bflags, EXTENT_BUFFER_WRITEBACK);
+-}
+-
+ static void set_btree_ioerr(struct page *page)
+ {
+ 	struct extent_buffer *eb = (struct extent_buffer *)page->private;
+-- 
+cgit v1.2.1-1-g437b
+ 
diff --git a/PKGBUILD b/PKGBUILD
index db77ec4..18f510e 100644
--- a/PKGBUILD
+++ b/PKGBUILD
@@ -42,6 +42,7 @@ source=(
         https://www.kernel.org/pub/linux/kernel/v5.x/linux-$pkgver.tar.{xz,sign}
         0001-add-sysctl-to-disallow-unprivileged-CLONE_NEWUSER-by.patch
         0002-ZEN-Add-CONFIG-for-unprivileged_userns_clone.patch
+        0003-Btrfs-fix-unwritten-extent-buffers-and-hangs-on-future-writeback-attempts.patch
         ck_remove-excess-extraversion.patch
         linux-ck-patch-${_majver}-${_ckpatchversion}.xz::http://ck.kolivas.org/patches/5.0/${_majver}/${_majver}-${_ckpatchversion}/${_ckpatch}.xz
         graysky_bdver2-hotfix.patch
@@ -57,6 +58,7 @@ sha256sums=('c64d36477fee6a864a734ec417407768e60040a13f144c33208fa9622fd0ce8c'
             'SKIP'
             '91fafa76bf9cb32159ac7f22191b3589278b91e65bc4505cf2fc6013b8037bf3'
             '63e4378e69e2f23ed87af32a4951477a6d82d4ac0de2295db46502c8120da9d9'
+            '940719452e133a7350dd5efea974190af07eb365a6a60d117b76efca4ca53e7a'
             '152c551bd03e92d6458b0d9f352c97eb058014ab019fa213167a6f8178bd6d71'
             'f1abc13a8d859fbf6350040e45d7f04ad551a6d39f113ba96fbbd820118c0e36'
             'c5405139aa0a90a6f68f6a13e066a2bd0600c970f9f525cd3aa114b044a7f73b'
@@ -87,6 +89,7 @@ prepare() {
   msg2 "Applying hotfixes"
   patch -p1 -i ../0001-add-sysctl-to-disallow-unprivileged-CLONE_NEWUSER-by.patch
   patch -p1 -i ../0002-ZEN-Add-CONFIG-for-unprivileged_userns_clone.patch
+  patch -p1 -i ../0003-Btrfs-fix-unwritten-extent-buffers-and-hangs-on-future-writeback-attempts.patch
 
 
   # ck hotfixes